(1) Field of the Invention
The present invention relates to silicon nitride sintered bodies having high strength at high temperatures and a process for manufacturing such sintered bodies through sintering at atmospheric pressure or sintering under a pressurized atmosphere.
(2) Related Art Statement
Heretofore, silicon nitride sintered bodies to which is added an oxide of an element in Group III.sub.a including a rare earth oxide or oxides as well as processes for manufacturing such sintered bodies have been proposed. For instance, Japanese Patent Publication No. 48-7,486 discloses a process for manufacturing silicon nitride sintered bodies, which comprises mixing not less than 85 mol% of Si.sub.3 N.sub.4 and not more than 15 mol% of at least one kind of an oxide selected from the oxides of elements in Group III.sub.a, molding the resulting mixture and then sintering the molding in a non-oxidative atmosphere. Japanese Patent Publication No. 49-21,091 discloses silicon nitride sintered bodies consisting of at least 50% by weight of Si.sub.3 N.sub.4, not more than 50% by weight of at least one kind of an oxide selected from Y.sub.2 O.sub.3, and oxides of La series elements, and from 0.01 to 20% by weight of Al.sub.2 O.sub.3.
However, silicon nitride sintered bodies having high strength at high temperatures cannot be obtained merely by adding a rare earth element or elements into silicon nitride. Although densification is promoted by the addition of Al.sub.2 O.sub.3 to Si.sub.3 N.sub.4, an intergranular phase has a low softening point so that strength at high temperatures is largely lowered.
As processes for producing silicon nitride sintered bodies having high strength at high temperatures, for instance, Japanese Patent Publication No. 59-35,867 discloses a method for producing silicon nitride sintered bodies in which not less than 2% by weight of La.sub.2 O.sub.3 and not less than 2% by weight of Y.sub.2 O.sub.3 are added in a total amount of not less than 10% by weight. Japanese Patent Application Laid-open No. 55-3,397 discloses a process for manufacturing silicon nitride sintered bodies in which an intergranular phase is crystallized by adding SiO.sub.2 and an oxide or oxides of a rare earth element or elements.
Further, U.S. Pat. No. 4,234,343 discloses silicon nitride sintered bodies which contain Yb, Y, Er or Dy and Sc, Al, Cr, Ti, etc. and in which an intergranular phase is crystallized into Si.sub.2 N.sub.2 O and M.sub.2 Si.sub.2 O.sub.7 (in which M is Yb, Y, Er, Dy, Sc, Al, Co or Ti).
As silicon nitride sintered bodies having high density and high strength which can be produced by a pressureless sintering process, i.e., sintering at atmospheric pressure, enabling sintering of complicated shapes of green bodies, for instance, Japanese Patent Application Laid-open No. 59-174,577 discloses silicon nitride sintered bodies to which from 5 to 30% by weight of two or more kinds of oxides selected from oxides of Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu are added.
However, in any of the above-mentioned conventional techniques, although silicon nitride sintered bodies having high strength at high temperatures can be obtained by hot pressing, silicon nitride sintered bodies having such high strength at high temperatures as obtained in the case of the hot pressing cannot be obtained by pressureless sintering or sintering under a pressurized atmosphere and high strength exceeding 500 MPa at high temperatures of around 1,400.degree. C. cannot be expected.